Fix gcc9 warning in MagneticField

[mrodozov]

PR description:

Fixes warning in MagneticField

PR validation:

Builds without warning

[cmsbuild]

The code-checks are being triggered in jenkins.

[mrodozov]

please test

[cmsbuild]

+code-checks

Logs: https://cmssdt.cern.ch/SDT/code-checks/cms-sw-PR-27853/11591

• This PR adds an extra 12KB to repository

[cmsbuild]

The tests are being triggered in jenkins.
https://cmssdt.cern.ch/jenkins/job/ib-run-pr-tests/2157/console Started: 2019/08/23 19:09

[cmsbuild]

A new Pull Request was created by @mrodozov (Mircho Rodozov) for master.

It involves the following packages:

MagneticField/VolumeBasedEngine

@perrotta, @cmsbuild, @slava77 can you please review it and eventually sign? Thanks.
@namapane this is something you requested to watch as well.
@davidlange6, @slava77, @fabiocos you are the release manager for this.

cms-bot commands are listed here

[cmsbuild]

+1
Tested at: ae443b0
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-8ec6aa/2157/summary.html

[cmsbuild]

Comparison job queued.

[cmsbuild]

Comparison is ready
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-8ec6aa/2157/summary.html

Comparison Summary:

• No significant changes to the logs found
• Reco comparison results: 4 differences found in the comparisons
• DQMHistoTests: Total files compared: 34
• DQMHistoTests: Total histograms compared: 2939508
• DQMHistoTests: Total failures: 2
• DQMHistoTests: Total nulls: 0
• DQMHistoTests: Total successes: 2939165
• DQMHistoTests: Total skipped: 341
• DQMHistoTests: Total Missing objects: 0
• DQMHistoSizes: Histogram memory added: 0.0 KiB( 33 files compared)
• Checked 142 log files, 14 edm output root files, 34 DQM output files

[slava77]

IIUC, the behavior before this PR is that the default constructor MagneticField::MagneticField() is called here.
The proposed update will change the old previous behavior.

I see no differences in the comparisons, which probably means that the copied field is probably never used in a way where the values in the base would matter.

@VinInn looking at cffc1ad
it may seem like the copy constructor was supposed to be called, but it was not done.
Please check and clarify.

@namapane are there any other use cases where this change in behavior would matter?

[namapane]

There is no real member in the base class MagneticField except for the nomial value which is computed on the fly the first time it is requested. The fix does not change anything as far as I can tell, except for actually avoiding recomputing the nominal value on the copy if that is requested (which is very rarely the case).
In short: the fix is correct. Thanks anyhow for the heads-up.

[slava77]

Thank you for checking and confirming that this PR is OK

[slava77]

+1

for #27853 ae443b0

• code changes are in line with the PR description and the follow up check in Fix gcc9 warning in MagneticField #27853 (comment)
• jenkins tests pass and comparisons with the baseline show no (relevant) differences

[cmsbuild]

This pull request is fully signed and it will be integrated in one of the next master IBs (tests are also fine). This pull request will now be reviewed by the release team before it's merged. @davidlange6, @slava77, @smuzaffar, @fabiocos (and backports should be raised in the release meeting by the corresponding L2)

[kpedro88]

+1